1. Field of the Invention
This invention relates to an optical fiber or an optical waveguide. More particularly, the present invention relates to a so-called W-type optical waveguide comprising a first transparent dielectric center layer having a relatively high refractive index, a second transparent dielectric intermediate layer having a relatively low refractive index and a third transparent dielectric cladding layer having an intermediate refractive index.
2. Description of the Prior Art
In recent years, transmission loss of so-called optical fiber for optical trnsmission has been reduced remarkably and practical applications of an optical communication system are now believed to be merely a matter of time.
In utilizing optical fibers for video communications which is believed to be one of the most significant fields of their application, however, there are several problems yet to be solved.
From the viewpoint of economy of the communication system, for instance, it is of utmost importance to make the best of the low transmission loss of the optical fiber. To accomplish this object, the transmission bandwidth of the optical fiber must be broadened. That is to say, the transmission band of the optical fiber is determined primarily by the group delay differences between the number of transmission modes used, and the group delay differences increase in proportion to the transmission distance. Extension of the transmission distance to take advantage of minimization of the transmission loss of the optical fiber therefore results in restriction of the transmission bandwidth.
For the above-mentioned reason, a given transmission line is evaluated in terms of the product of the width of its transmission bandwidth and the distance of transmission.
A conventionally known optical fiber of the type (usually referred to as a "single clad fiber") has a two-layer construction comprising a center layer of relatively high refractive index (which is usually referred to as the "core") and a cladding layer of a relatively low refractive index. This type of optical fiber, however, has the problem that its low transmission loss characterizing feature cannot be utilized to a maximum extent because of the following two points.
Namely, if the difference between the two refractive indices are reduced so as to thereby broaden the range of available bands (that is to say, to minimize the group delay difference), the transmission capability of the optical fiber also is reduced to such an extent that the optical fiber is no longer practical. If the difference between the two refractive indices is increased, on the other hand, in order to enhance the transmission capability, the group delay difference is also increased, thereby narrowing the bandwidth. Because of these problems, the conventional optical fiber of the two-layer configuration cannot be used satisfactorily for a long distance transmission.